Project Summary The quality of close friendships during adolescence is related to physical and mental health over a decade later. This outsized and lasting influence is consistent with theories that adolescence is a sensitive period for social development. Intervening on social relationships during this period may improve adolescent mental health and mitigate the lifespan disease burden associated with poor social relationships. This public health strategy may be particularly beneficial for adolescent girls, a population with specific vulnerabilities and biopsychosocial pathways to depression. This project investigates these pathways by examining close adolescent friendships in three waves of data from a longitudinal study of adolescent females (initial N = 174, initial ages 10-13, 18 months between waves). We focus on self-disclosure, a critical behavior for forming and maintaining feelings of intimacy within friendships. Our prior work with an fMRI adaptation of the Self-Disclosure Task suggests that adolescents find disclosing to a close friend to be intrinsically rewarding, and that neural responses during disclosure decisions are related to friendship quality and feelings of being supported. The guiding framework of this project is that intimacy and affiliation within close friendships may serve as protective and/or risk factor for the emergence of depression via their influence on the functioning of a cortico-striatal circuit during specific phases of pubertal development. To test this framework, the specific aims of this proposal are (1) to predict the emergence of depressive symptoms from close friendship quality and neurobehavioral indices of self-disclosure and, (2) to probe a neurodevelopmental mechanism relating patterns of neural responses during self-disclosure processes to changes in the brain?s resting-state functional architecture over time. (3) A third exploratory aim examines age and puberty as moderators of these associations. Completion of these research aims will result in a body of work that examines the influence of close friendship quality and self-disclosure processes on psychopathology and neurodevelopment across time and identifies timing windows during which these effects might be more pronounced. Dr. Jennifer Pfeifer will be the primary training mentor overseeing the project due to her extensive knowledge of adolescent social development and developmental neuroimaging. Additional mentors will take the lead on each of three training goals: (1) advanced methods for longitudinal developmental neuroimaging (Dr. Kate Mills), (2) biopsychosocial models of adolescent internalizing problems (Dr. Nick Allen), and (3) open and reproducible neuroscience (Dr. Rob Chavez). Both the primary mentor (Dr. Pfeifer) and co- sponsor (Dr. Mills) will provide substantive mentorship and hands-on support across research and training aims. This coordinated didactic, experiential, and professional training support my long-term goal of becoming a developmental scientist investigating the nature of sensitive periods for social development to identify malleable factors and sensitive timing windows for adolescent psychopathology.